1. Technical Field
The present invention relates to a core of a light water reactor and a fuel assembly and more particularly to a core of a light water reactor and a fuel assembly preferably applied to a boiling water reactor.
2. Background Art
When actinide nuclide having many isotopes burns in a core in a state that it is enriched in a nuclear fuel material in fuel assemblies loaded in a core of a light water reactor, the actinide nuclide transfers successively among the isotopes by nuclear reaction such as neutron capture and nuclear fission. In the actinide nuclide, since odd-numbered nucleus that has a large nuclear fission cross section with respect to a resonance and thermal neutrons, and even-numbered nucleus that undergoes fission only for fast neutrons are present, in general, isotopic composition in the actinide nuclides included in the fuel assembly largely change as the actinide nuclides burn. It is known that this isotopic composition change depends on the neutron energy spectrum at the position at which the fuel assembly is loaded in the core.
Current light water reactor uses slightly enriched uranium as nuclear fuel. However, since the natural uranium resource is finite, it is necessary to successively replace fuel assemblies used in the light water reactor with recycle fuel assemblies including a nuclear fuel material which is formed by enriching depleted uranium, which is a residual after uranium enrichment, natural uranium, thorium, or degraded uranium with the transuranic nuclide (hereinafter referred to as TRU) extracted from the spent fuel assemblies of the light water reactor. Further, depleted uranium, natural uranium, thorium, degraded uranium, and TRU are referred to as a nuclear fuel material. The fuel assembly having the nuclear fuel material is loaded in the core of the light water reactor. It is desirable that U-233 newly generated by absorbing neutrons by the TRU and thorium are recycled as a useful resource over a very long period during which a commercial reactor is predicted to be necessary and during the period, the quantities of TRU and U-233 always increase or be maintained almost constant.
In the light water reactor occupying most of the current commercial reactors, the technology of realizing a breeder reactor for increasing or maintaining almost constant the quantity of fissionable Pu while the nuclear fuel material burns, is described in Japanese Patent 3428150 (U.S. Pat. No. 5,812,621) and R. TAKEDA et al., Proc. of International Conference on Evaluation of Emerging Nuclear Fuel Cycle Systems. GLOBAL '95 Versailles, France, September, 1995, P. 938. In the light water reactor realizing the breeder reactor described in Japanese Patent 3428150 and R. TAKEDA et al., Proc. of International Conference on Evaluation of Emerging Nuclear Fuel Cycle Systems. GLOBAL '95 Versailles, France, September, 1995, P. 938, a plurality of fuel assemblies, each of which has a hexagonal transverse cross section, are disposed in the core, each fuel assembly being formed by closely arranging a plurality of fuel rods in a triangular grid. In the core of this light water reactor, the amount of water around the fuel rods is lessened due to the close arrangement of the fuel rods, and thereby the proportion of resonant energy neutrons and fast energy neutrons are increased. In addition, the height of a mixed oxide fuel section of the TRU is reduced and blanket zones loaded with depleted uranium are disposed above and below the mixed oxide burning part so as to maintain a negative void coefficient, which is a safety criterion. The core is formed in two stacked stages by applying the concept of a parfait-type core described in G. A. Ducat et al., “Evaluation of the Parfait Blanket Concept for Fast Breeder Reactors”, MITNE-157, January, 1974, thereby a breeding ratio of 1 or more is ensure, keeping the economy.
To recycle TRU, the reprocessing of spent fuel is indispensable. Due to a fear that consumer TRU is diverted to weapons of mass destruction, there has been an increasing demand for nuclear non-proliferation and thereby restrictions on TRU recycling have been severe.
Further, it is certain that an electric power generating system superior to a fission reactor is put into practical use on some day in the future. At that time, the value of TRU is lowered from a very useful fuel equivalent to enriched uranium to a cumbersome long-life waste material. Therefore, in order to spread a light water reactor using uranium as nuclear fuel widely in the world, to prepare the disposal method of TRU remaining in the spent nuclear fuel, that is, a TRU burner reactor for fissioning the TRU to a fission product is a most important object in the nuclear power development.
Japanese Patent Laid-Open No. 2008-215818 and R. TAKEDA et al., Proc. of International Conference on Advanced Nuclear Fuel Cycles and Systems. GLOBAL '07 Boise, USA, September, 2007, P. 1725, propose a light water breeder reactor for keeping the isotopic composition of the TRU almost constant and recycling the TRU and the TRU burner reactor for permitting the TRU to fission in order to realize multiple-recycling for repeatedly executing the recycling for reusing the TRU obtained by reprocessing the spent nuclear fuel as new nuclear fuel.
The light water breeder reactor has a core for recycling nuclear fuel in a state that the TRU quantity is kept constant or is increased and loading the fuel assemblies increasing the burn-up and nuclear proliferation resistance. The TRU burner reactor is a nuclear reactor for successively gathering the TRU while decreasing the TRU recovered by reprocessing the nuclear fuel by nuclear fission and permitting all the TRU to fission excluding the last one core in order to prevent the TRU from becoming a long-life radioactive waste material, when the light water reactor reaches an ending time of the mission.
The light water reactor described in R. TAKEDA et al., Proc. of International Conference on Evaluation of Emerging Nuclear Fuel Cycle Systems. GLOBAL '95 Versailles, France, September, 1995, P. 938 and R. TAKEDA et al., Proc. of International Conference on Advanced Nuclear Fuel Cycles and Systems. GLOBAL '07 Boise, USA, September, 2007, P. 1725 for recycling the TRUs recovered from the spent nuclear fuel, to meet the design criteria for abnormal transient and accidents, keeps the TRU quantity constant with a sufficient safety margin, effectively uses the TRUs as seeded fuel, and burns all depleted uranium, thereby realizes long-term stable energy supply. Furthermore, such a recycle reactor can be realized as permits all the TRUs to fission and preventing the TRUs from becoming a long-life waste material when the nuclear fission reactor ends the mission and thus the TRUs become unnecessary.